Vol. 8, No. 1, January 1969 THEFLUOROSVLFURIC ACIDSOLVENT SYSTEM 63 enzyme was not created. One particularly active cat- Acknowledgment.-This research was carried out alyst was tested, but the mode of catalysis is not yet under Grant GM 11989 from the National Institutes of understood. Health. CONTRIBUTIONFROM THE DEPARTMEKTOF CHEMISTRY, MCMASTERUNIVERSITY, HAMILTON, OKTARIO, CANADA The Fluorosulfuric Acid Solvent System. VI. Solutions of Phosphorus, Arsenic, Bismuth, and Niobium Pentafluorides and Titanium Tetrafluoride BY R. J. GILLESPIE, K. OUCHI, AND G. P. PEZ Received September 4, 1968 Conductivity measurements on solutions of PFs, AsF5, BiF5, NbF5, PFs-SOa, NbFj-SOa, and AsFg-S03 are reported. Con- ductometric titrations have been carried out on solutions of AsFj, BiF5, and AsF5.SOa. The results are compared with those obtained previously for SbF5 and SbF5-SO3. It is concluded that acid strength increases in the order: PFs - NbF5 < TiF4 - AsFr < BiF6 < AsFd(S03F) < SbF5 < AsFz(SO~F)~< SbFz(S03F)s. It was shown in part 111 of this series that antimony any of them are stronger acids than SbFS. The effect pentafluoride is a rather weak acid of the fluorosulfuric of sulfur trioxide on the acidity of some of the systems acid solvent system ionizing according to the equation was also studied. SbFj + 2HS03F +H[SbFj(SOsF)] + HSOsF Experimental Section HzS03F+ + SbF5(SOsF)- Niobium Pentafluoride.-Technical grade Ozark-Mahoning the acid H [SbF5(S03F)] having a dissociation constant material was purified by triple distillation under vacuum at 100-110”, mp 79-81’. K = 3.7 X 10-3 mol kg-l. Although antimony penta- Bismuth Pentafluoride.-Bismuth trifluoride was fluorinated in fluoride is not fully ionized, these solutions have an ex- a flow system at 500°.4 The crude BiFs was purified by sublima- tremely high acidity because of the rather high concen- tion under vacuum in an apparatus made entirely of Vycor glass tration of the fluorosulfuric acidium ion HzS03F+ and which is not attacked by BiF5 at -100”. they have been extensively employed for studies of the Phosphorus Pentafluoride.-Dry diazonium hexafluorophos- phate (Phosfluogen A, Ozark Mahoning Co.) was thermally protonation of very weak bases and for the formation of decomposed at 130’ and the resulting PFj was purified by trap- stable solutions of carbonium ions2 Because of the to-trap distillation on a vacuum line. great utility of this medium for the production of new Arsenic Pentafluoride.-The material obtained from a cylinder cationic species, it has been termed “magic acid” by Olah supplied by the Allied Chemical Co. was purified by trap-to-trap and his coworker^.^ It was also shown in part 111 that distillation on a vacuum line. Titanium Tetrafluoride.-Pure material obtained from the addition of sulfur trioxide to this system still further in- Allied Chemical Co. and the Titanium Alloy Manufacturing Co. creases the acidity as SbF6 is replaced by the fluorosul- was used after rigorous drying. fates SbF4(S03F), SbF3(S03F)z1and SbF&03F)3 which Fluorosulfuric acid and sulfur trioxide were purified as de- increase in acid strength with increasing number of scribed previously.lZ6 Conductivity measurements were made fluorosulfate groups. Indeed the trifluorosulfate SbF2- with the apparatus and technique described previously.5 (S08F)s forms the strong acid H [SbFz(S03F)4J in solu- Results and Discussion tion in fluorosulfuric acid which is fully ionized accord- The results of conductivity measurements on solu- ing to the equation tions of phosphorus, arsenic, bismuth, and niobium H[SbFn(SOaF)r] + HSOIF ---j. SbFz(S03F)d- + HnS03F’ pentafluorides and titanium tetrafluoride are given in Although this HSO~F-S~FS-SO~medium has not been Table I and Figure 1. It may be seen in the figure nearly so extensively exploited as the “magic acid” that all of the fluorides give much smaller conductivities (HS03F-SbF6) it is certainly considerably more acidic, than SbF5, and, therefore, if they behave as acids at all, at least at low SbFS concentrations (0-0.3 m SbF5). they are clearly considerably weaker than antimony pen- The object of the work described in this paper was to tafluoride. Niobium and phosphorus pentafluorides investigate the behavior of other fluorides, primarily give an almost negligible increase in the conductivity pentafluorides, in fluorosulfuric acid, to see if they ex- and must therefore be essentially nonelectrolytes. ,4r- hibit acid behavior and, in particular, to ascertain if senic and bismuth pertafluorides and titanium tetra- fluoride all cause a rather similar small increase in the (1) R. C. Thompson, J. Barr, R. J. Gillespie, J. B. Milne, and R. A. (4) G. Brauer, “Handbook of Preparative Inorganic Chemistry,” 1701. 1, Rothenbury, Inovg. Chem., 4, 1641 (1965). 2nd ed, Academic Press, New York, N. Y., 1963, p 202. (2) R. J. Gillespie, Accounts Chem. Res., 1, 202 (1968). (5) J. Barr, R. J. Gillespie, and R. C. Thompson, Inovg. Chem., 3, 1149 (3) G. A. Olah and R. H. Schlosberg, J. Am. Chem. Soc., 90, 2726 (1968). (1964). 64 R. J. GILLESPIE,K. OUCHI,AND G. P. PEZ Inorganic Chemistry TABLEI conductivity. The conductivity of the bismuth penta- CONDUCTIVITIESOF SOLUTIONS IN FLUOROSULFURIC ACID^ fluoride solutions is slightly greater than that of the ar- lo%, lopK, senic pentafluoride solutions and it is presumably a 10% ohm-' cm-1 10% ohm-' cm-1 slightly stronger acid but both are considerably weaker BiF5 AsF5 than antimony pentafluoride. By analogy with the 0.5842 2.911 2.27 5.092 established behavior of SbF6,we presume that AsFj and 1.832 5.861 5.09 6.60 BiF5 ionize as acids according to the equations 2.893 7.569 6.17 10.06 5.542 11.00 9.29 10.79 AsF; f 2HS03F HzS03F' + AsFs(S03F)- 10 * 92 16.60 11.56 14.24 17.82 20.44 13.48 14.72 BiFj + 2HS03F H2S03F- + BiFj(SO1F)- 26.83 26.58 25.87 17.68 The mode of ionization of TiF4 cannot be predicted hTbFj PF5 with such certainty; possibly it ionizes as 2.30 3.081 5.19 2.22 TiFd ZHSO3F HzS03F+ f TiF4(S03F)- 3.20 2.915 6.11 4.02 f J' 5.46 3.350 TiF4 In any case its solubility appears to be quite small (4 X 6.9 3.72 0.392 2.039 11.1 4.72 1.375 4.332 m at 25"). The proposed acid behavior of BiFj and AsFj was AsF~.SO3 2.546 5.562 confirmed by conductometric titration with the base 1.09 47.45 3.803 7.032 KS03F (Figure 2).l In such a titration a strong acid, AsFj.%SOs (W >/ 3) 5.98 65.34 9.15 90.48 12.22 110.4 14.77 124.8 19.04 148.7 a Conductivities of the HSOlF solvent ranged from 1.5 to 2.2 X 14 SbF5.3S0, 01 I I I ' ' I ' ' I I ' " 0 .I .2 .3 .4 .5 .6 .7 .E .9 1.0 1.1 1.2 Mole Ratio KSO,F/MF, (MF,.SO,F) Figure 2.-Conductometric titrations of some acids with KSOIF in fluorosulfuric acid. e.g., H [SbF2(S03F)4],gives a minimum conductivity at the mole ratio KS03F/acid = 1. Weaker acids give a minimum at mole ratio KSOsF/acid < 1. For BiFj a minimum conductivity was obtained at a mole ratio KSO3F/BiF5 = 0.06 while for AsFj the minimum was not very clear but appeared to be approximately at the very small mole ratio KS03F/AsF5 = 0.01. Thus the acid behavior of BiFj and AsFj is confirmed as is also m lo2 the fact that BiF5 is a slightly stronger acid than AsF5. Figure 1.-Conductivities in fluorosulfuric acid: A, TiK; No conductometric titrations could be carried out with U, AsF5; V, PFs; 0 , NbF5. The asterisk signifies that these solu- PF5 and NbF5. tions had the composition AsFs.nSO8, n > 3, but excess SO8 over the composition AsFj.3SOa has very little effect on the Since the addition of sulfur trioxide has a marked conductivity. effect on the acidity of a solution of SbFj in fluorosul- Vol. 8, No. 1, January 1969 PMRLINE BROADENINGBY VANADYLION 65 furic acid, the effect of sulfur trioxide on some of these sulfate group appears generally to cause an increase in acid systems was also investigated. Sulfur trioxide the acceptor strength, but unfortunately the maximum was found to have no effect on the conductivities of acidity that is, in principle, possible cannot be obtained solutions of NbF5 and PF5 both of which have negli- as the pentafluorosulfates do not appear to be stable. gible acidic properties, but it was found to have a con- At the present time therefore it appears that SbF2- siderable effect on the conductivities of AsFh solutions. (S03F)3 is the strongest known acceptor for fluorosul- For example, at a concentration of 0.11 m, addition of fate ion and that HS03F-SbF5-SO3, where n 2 3, is sulfur trioxide to give the mole ratio S03/AsF5 = 1 in- the strongest proton-donating medium, ;.e., the creased the conductivity by more than three times and strongest acid in the Br#mted sense. The increase in further addition of sulfur trioxide to give the mole ratio acidity on replacing fluoride by fluorosulfate may be S03/AsF5 = 3 increased the conductivity by more than reasonably attributed to the greater dispersal of nega- seven times (Figure 2). The conductivity was not tive charge that is made possible by the introduction of much affected by larger amounts of sulfur trioxide, and fluorosulfate groups. we presume that, as for SbF6, arsenic pentafluoride It should be emphasized that the above conclusions combines with a maximum of 3 mol of sulfur trioxide to concerning the acidity of the HS03F-SbF,-nS03 and give ASF~(SO~F)~.Thus AsF5.SO3 which we presume HS03F-AsF5-nS03 systems refer only to rather dilute to be AsF4.S03F is a somewhat weaker acid than SbFs solutions of SbF5.